Automatic analyzers are used for detecting particular biogenic substances, chemical substances, etc. contained in a sample (blood, blood serum, urine, etc.) especially in the fields of medical care, biotechnology, etc. Among such automatic analyzers, there is an automatic analyzer comprising a suction nozzle, a syringe which is connected to the suction nozzle by a channel and operates to cause a liquid to be sucked into the suction nozzle, and a detector of the flow cell type which is arranged in the middle of the channel (hereinafter referred to as a “detection channel”) connecting the suction nozzle and the syringe (see Patent Literature 1, for example). In this type of automatic analyzer, a reaction solution in which the sample and a reagent have undergone a reaction is sucked into the flow cell through the suction nozzle, and particular biogenic substances, chemical substances, etc. contained in the sucked reaction solution are detected by the detector.